We are what we repeatedly do. Excellence, then, is not an act, but a habit.

Source: Will Durant, The Story of Philosophy: The Lives and Opinions of the World's Greatest Philosophers (1926), reprinted in Simon & Schuster/Pocket Books, 1991, ISBN 0-671-73916-6], Ch. II: Aristotle and Greek Science; part VII: Ethics and the Nature of Happiness: "Excellence is an art won by training and habituation: we do not act rightly because we have virtue or excellence, but we rather have these because we have acted rightly; 'these virtues are formed in man by his doing the actions'; we are what we repeatedly do. Excellence, then, is not an act but a habit: 'the good of man is a working of the soul in the way of excellence in a complete life... for as it is not one swallow or one fine day that makes a spring, so it is not one day or a short time that makes a man blessed and happy'" (p. 76). The quoted phrases within the quotation are from the Nicomachean Ethics, Book II, 4; Book I, 7. The misattribution is from taking Durant's summation of Aristotle's ideas as being the words

We are what we repeatedly do, therefore excellence is not an act, but a habit.